The present invention relates to a capacitive pressure sensor having a diaphragm structure and designed to capacitively detect a change in pressure under measurement, and a method of manufacturing the same.
FIG. 4 shows the arrangement of a capacitive pressure sensor of this type, e.g., disclosed in Japanese Patent Laid-Open No. 63-298130. Referring to FIG. 4, a diaphragm 45 having an upper movable electrode 44 is formed through a hollow portion 43 on a lower stationary electrode 41 formed on the upper surface of a photosensitive glass substrate 42. A pressure introducing fine hole 46 communicating with the hollow portion 43 is formed through the photosensitive glass substrate 42, thus forming a diaphragm structure.
FIGS. 5A to 5D show a method of manufacturing a capacitive pressure sensor of this type, e.g., disclosed in Japanese Patent Laid-Open No. 63-208735. As shown in FIG. 5A, a thin film 47 consisting of a solution containing a pyrolysis material is formed on a photosensitive glass substrate 42 having a surface on which a lower stationary electrode 41 is formed. Thereafter, as shown in FIG. 5B, a metal deposition film 48 is formed on the thin film 47. After the thin film 47 is patterned to have a specific shape by using the metal deposition film 48 as a protective mask, as shown in FIG. 5C, a metal film is stacked on the metal deposition film 48 and its peripheral portion on the photosensitive glass substrate 42, thus forming a diaphragm 45, as shown in FIG. 5D. Subsequently, a hollow portion 43 equivalent to the one shown in FIG. 4 is formed by removing the pyrolysis material of the thin film 47 by pyrolytically decomposing it, thus forming a diaphragm structure.
In general, a stepped portion of a film formed by a thin-film forming process is thinner and lower in quality than a non-stepped portion. In addition, it is known that the thickness and quality of a stepped portion are difficult to control compared with a non-stepped portion.
Furthermore, when a diaphragm receives a pressure, a maximum stress is produced at an end portion of a movable portion. As a result, the mechanical characteristics of the diaphragm end portion greatly affect the mechanical characteristics of the overall diaphragm, especially the pressure displacement conversion characteristics (the flexibility of the diaphragm) and the excessive pressure resistance characteristics (the durability of the diaphragm). Therefore, when a diaphragm portion is to be formed of a thin film, no stepped portion is preferably formed at an end portion of a movable portion of the diaphragm.
In practice, however, stepped portions are formed at end portions of the movable portion of the diaphragm, as shown in FIG. 4. Therefore, the mechanical characteristics of the movable portion of the diaphragm are degraded. Moreover, when diaphragms are mass-produced, the mechanical characteristics of the movable portions may vary. This is because stepped portions are inevitably formed at end portions of the movable portions of diaphragms manufactured by the conventional manufacturing method, as shown in FIGS. 5A to 5D.